1. Field of the Invention
This invention relates to a composite twist-spun yarn of the type having a central “hard” core covered with a dual-spun fiber covering, as well as to fabrics woven or knitted from the composite dual core-spun yarn, and to a method and a device for production of the yarn.
2. Description of Related Art
The invention is particularly concerned with improvements in twist-spun yarns that are substantially inextensible, i.e. where the central hard core has an elongation at break less than 50%. Elongation at break of a yarn specimen is the increase in length produced by the breaking force, expressed as a percentage of the original nominal length. All values of elongation at break in the present disclosure are those established according to the methodology based ISO 2062, according to which a specimen of yarn is extended until rupture by a suitable mechanical device and elongation at break are recorded. A constant rate of specimen extension of 100% per minute (based on the specimen length) is used. Although ISO 2062 makes reservations about its applicability to certain yarns, its method is adequate for determining if any yarn has an elongation at break below or above 50%.
Twist spun yarns with a central core covered with a dual-spun fiber covering are produced by bringing together two fiber slivers to form a spinning triangle, feeding the core in the spinning triangle between the two fiber slivers with the latter at an angle to the core, and spinning the brought-together fiber slivers around the core with an S or Z twist that is the same as or opposite to that of the core.
This so-called Siro-core-spun process—which has the advantage of being a “one-step” spinning process—has been successful in particular for producing stretchable yarns that are widely used for manufacturing stretch fabrics. These stretch yarns have elastane cores made for example of the polyurethane-elastane available from E. I. du Pont de Nemours and Company, Wilmington, Del., U.S.A., under the trademark LYCRA®.
Elastane cores typically have an elongation at break of 400% or more. During the spinning process the elastane core is drafted between 250% and 350%, such that the elasticity of the core “takes up” the fiber covering, leading to the production of composite elastic yarns with consistent stretch and coverage by the fiber covering. However, when the Siro-core-spun process is applied to substantially inelastic cores (elongation at break less than 50%, usually well below 50%, and rarely exceeding 40%), problems arise. During the spinning process, it is difficult to guide the inextensible core to the convergence point of the spinning triangle, and the core is liable to jump and break. In the resulting composite twist spun yarns, the core tends to emerge to the surface at points along the yarn, leading to a “low” coverage of the core. The maximum achievable coverage of the inextensible core is about 70%. Methods of estimating the core coverage are described below. When the core and covering are of contrasting colors, this leads to a speckled appearance in fabrics woven or knitted from the yarn, known as “Chiné”, which is not always wanted. For these reasons, the Siro-core-spun process has not been used for inelastic hard cores to a great extent and, when it is, special precautions need to be taken and there are serious limitations in the produced yarn.
A different process for spinning twist-spun yarns with a substantially inextensible central core has been proposed in European Patent 0 271 418. This discloses a process for producing a composite yarn by feeding the core, in particular an aramid core, with the core's torsion coefficient appreciably less than its critical torsion coefficient, and twisting the covering fibers on the core during the spinning operation such that the total torsion coefficient of the yarn is less than its critical torsion coefficient. More precisely, the torsion coefficient of the core (discussed further below) is equal to the value of the critical torsion coefficient of the yarn less the value of the total torsion coefficient of the composite yarn multiplied by the proportion of the core yarn in the composite yarn. The process of EP 0 271 418 has the disadvantage that the produced core yarn necessarily has a resulting torque. To obtain a substantially torqueless final yarn, two of the covered yarns must be assembled by twisting them together in opposite directions, as will be explained below in connection with FIG. 3. This implies a two step spinning process, which is less attractive.